Pohlmann, AThe Compact Disc@, THE COMPUTER MUSIC & DIGITAL AUDIO SERIES, Volume 5. The above-mentioned audio series, which was published by A-R Editions, Inc., in Madison, Wis., is, along with all volumes therein, incorporated by reference.
In analog recording, the recording medium (a tape) varies continuously according to the sound signal. In other words, an analog tape stores sound signals as a continuous stream of magnetism. The magnetism, which may have any value within a limited range, varies by the same amount as the sound signal voltage.
In digital recording, the sound signal is sampled electronically and recorded as a rapid sequence of separately coded measurements. In other words, a digital recording comprises rapid measurements of a sound signal in the form of on-off binary codes represented by ones and zeros. In this digital system, zeros are represented by indentations or pits in a disc surface, and ones are represented by unpitted surfaces or land reflections of the disc, such that a compact disc contains a spiral track of binary codes in the form of sequences of minute pits produced by a laser beam.
Music that is input to a digital recording and the requisite series of reproduction processes, must pass through the recording side of a pulse code modulation (PCM) system. A master recording of the music is stored in digital form on a magnetic tape or optical disc. Once the magnetic tape has been recorded, mixed and edited, it is ready for reproduction as a CD. The CD manufacturer then converts the master tape to a master disc, which is replicated to produce a desired number of CDs. At the end of the PCM system is the reproduction side, the CD player, which outputs the pre-recorded music.
If digital technology is used in all intermediate steps between the recording and reproduction sides of the PCM system, music remains in binary code throughout the entire chain; music is converted to binary code when it enters the recording studio, and stays in binary code until it is converted back to analog form when it leaves the CD player and is audible to a listener. In most CD players, digital outputs therefrom preserve data in its original form until the data reaches the power amplifier, and the identical audio information recorded in the studio is thereby preserved on the disc.
Optical Storage
The physical specifications for a compact disc system are shown in Prior Art FIG. 1. They were developed jointly by Sony and Philips, and are defined in the standards document entitled Red Book, which is incorporated herein by reference. The CD standard is also contained in the International Electrotechnical Commission standard entitled, Compact Disc Digital Audio System, also incorporated herein by reference. Disc manufacturers, as well as CD player manufacturers, generally use these specifications.
All disc dimensions, including those pertaining to pit and physical formations, which encode data, are defined in the CD standard. For example, specifications information on sampling frequency, quantization word length, data rate, error correction code, and modulation scheme are all defined in the standard. Properties of the optical system that reads data from the disc using a leaser beam are also defined in the standard. Moreover, basic specifications relevant to CD player design are located in the signal format specifications.
Referring to Prior Art FIG. 2, the physical characteristics of the compact disc surface structure are described. Each CD is less than 5 inches in diameter whose track thickness is essentially thinner than a hair and whose track length averages approximately 3 and a half miles. The innermost portion of the disc is a hole, with a diameter of 15 mm, that does not hold data. The hole provides a clamping area for the CD player to hold the CD firmly to the spindle motor shaft.
Data is recorded on a surface area of the disc that is 35.5 mm wide. A lead-in area rings the innermost data area, and a lead-out area rings the outermost area. Both lead-in and lead-out areas contain non-audio data used to control the CD player. Generally, a change in appearance in the reflective data surface of a disc marks the end of musical information.
A transparent plastic substrate comprises most of the CD=s 1.2 mm thickness. Viewing a magnified portion of the CD surface, as shown in Prior Art FIG. 2, the top surface of the CD is covered with a very thin metal layer of generally aluminum, silver or gold. Data is physically contained in pits impressed along the CD=s top surface. Above this metalized pit surface and disc substrate, lies another thin protective lacquer coating (10 to 30 micrometers). An identifying label (5 micrometers) is printed on top of the lacquer coating.
A system of mirrors and lenses sends a beam of laser light to read the data. A laser beam is applied to the underside of a CD and passes through the transparent substrate and back again. The beam is focused on the metalized data surface that is sandwiched or embedded inside the disc. As the disc rotates, the laser beam moves across the disc from the center to the edge. This beam produces on-off code signals that are converted into, for example, a stereo electric signal.
The Pit Track
Prior Art FIG. 3 shows a typical compact disc pit surface. Each CD contains a track of pits arranged in a continuous spiral that runs from the inner circumference to the outer edge. The starting point begins at the inner circumference because, in some manufacturing processes, tracks at the outer diameter of a CD are more generally prone to manufacturing defects. Therefore, CDs with shorter playing time provide a greater manufacturing yield, which has led to adoption of smaller diameter discs (such as 8 cm CD-3 discs) or larger diameter discs (such as 20 and 30 cm CD-Video discs).
Prior Art FIG. 4 shows a diagram of a typical track pitch. The distance between successive tracks is 1.6 micrometers. That adds up to approximately 600 tracks per millimeter. There are 22,188 revolutions across a disc=s entire signal surface of 35.5 millimeters. Hence, a pit track may contain 3 billion pits. Because CDs are constructed in a diffraction-limited manner—creating the smallest formations of the wave nature of light—track pitch acts as a diffraction grating; namely, by producing a rainbow of colors. In fact, CD pits are among the smallest of all manufactured formations.
The linear dimensions of each track on a CD is the same, from the beginning of a spiral to the end. Consequently, each CD must rotate with constant linear velocity, a condition whereby uniform relative velocity is maintained between the CD and the pickup.
To accomplish this, the rotational speed of a CD varies depending on the position of the pickup. The disc rotates at a playing speed which varies from 500 revolutions per minute at the center, where the track starts, to 200 revolutions per minute at the edge. This difference in speed is accounted for by the number of tracks at each position.
For example, because each outer track revolution contains more pits than each inner track revolution, the CD must be slowed down as it plays in order to maintain a constant rate of data. So, when the pickup is reading the inner circumference of the CD, the disc rotates at the higher speed of 500 rpm. And as the pickup moves outwardly towards the disc=s edge, the rotational speed gradually decreases to 200 rpm. Thus, a constant linear velocity is maintained, such that all of the pits are read at the same speed. The CD player constantly reads from synchronization words from the data and adjusts the disc speed to keep the data rate constant.
A CD=s constant linear velocity (CLV) system is significantly different from an LP=s system. A major difference stems from the fact that a turntable=s motor rotates at a constant velocity rate of 33⅓ grooves. This translates into outer grooves having a greater apparent velocity than inner grooves, probably explained by the occurrence that high-frequency responses of inner grooves are inferior to that of outer grooves. If a CD used constant angular velocity (CAV) as opposed to the CLV system, pits on the outside diameter would have to be longer than pits on the inner diameter of the disc. This latter scenario would result in decreased data density and decreased playing time of a CD.
Like constant linear velocity, light beam modulation is also important to the optical read-out system that decodes the tracks. See Prior Art FIG. 5. A brief theoretical discussion on the distinctions between pit and land light travel explains this point.
Generally, when light passes from one medium to another with a different index of refraction, the light bends and its wavelength changes. The velocity at which light passes is important, because when velocity is slow, the beam bends and focusing occurs. Owing to several factors, such as the refractive index, disc thickness and laser lens aperture, the laser beam=s size on the disc surface is approximately 800 Φm. However, the laser beam is focused to approximately 1.7 Φm at the pit surface. In other words, the laser beam is focused to a point that is a little larger than a pit width. This condition minimizes the effects of dust or scratches on the CD=s outer surface, because the size of dust particles or scratches are effectively reduced along with the laser beam. Any obstruction less than 0.5 ml are essentially insignificant and causes no error in the readout.
As previously noted, a CD=s entire pit surface is metalized. In addition, the reflective flat surface between each pit,(i.e. a land), causes almost 90 percent of laser light to be reflected back into the pickup. Looking at a spiral track from a laser=s perspective on the underside of a disc, as shown in Prior Art FIG. 5, pits appear as bumps. The height of each bump is generally between 0.11 and 0.13 Φm, such that this dimension is smaller than the laser beam=s wavelength (780 nanometers) in air. The dimension of the laser beam=s wavelength in air is larger than the laser=s wavelength (500 nanometers) inside the disc substrate, with a refractive index of 1.55. In short, the height of each bump is, therefore, one-quarter of the laser=s wavelength in the substrate.
Scientifically, this means that light striking a land will travel twice as far than light striking a bump. This discrepancy in light travel distance serve to modulate the intensity of a light beam. This allows data physically encoded on the disc to be recoverable by the laser.
Also, the pits and intervening reflective land on the disc=s surface do not directly designate ones and zeros. Rather, it is each pit=s edge, whether leading or trailing, that is a 1 and all areas in between, whether inside or outside a pit, that are designated as zeros. Still, each pit and reflective land lengths vary incrementally. The combinations of 9 different pit and land lengths of varying dimensions physically encode the data.
Error Correction
Error correction is one of the major advantages of digital audio storage media, such as compact discs, over analog media, like LPs. Error correction simply corrects the error.
When an LP is scratched, for instance, the grooves are irrevocably damaged, along with the information contained in them. On every replay of that record, there will be a click or pop when the damaged part of the groove passes beneath the needle.
This is not the case for CDs. The data on every disc is specially encoded with an error correction code. When a scratched CD is played, the CD player uses the error correction code to perform error correction every time the disc is played. Thus, it delivers the original undamaged data, instead of the damaged data.
Cross Interleave Reed-Solomon Code (CIRC)
As indicated above, error correction is essential to the success of digitized audio information. Otherwise, any digital recording, whether on tape or disc, would sound like a badly scratched LP.
The raw error rate from a CD is approximately 10−5 to 10−6; that is, about one error for ever 1 million bits. To put this in perspective, a disc will output over 4 million bits per second. So, while the raw error rate is impressive, the need for error correction is obvious.
With error correction, approximately 200 errors per second will be completely corrected. To achieve these results, each compact disc employs interleaving to distribute errors, and parity to correct them. Interleaving is the process of arranging data in time. Parity is a redundant error detection method in which the total number of binary ones (or zeros) is always even or odd. Interleaving and parity are the cornerstones of error correction.
The particular algorithm used for correcting errors in all compact disc systems is the Cross Interleave Reed-Solomon Code, (ACIRC@). In short, CIRC is a method of error detection and correction using data delay, rearrangement, and the Reed-Solomon coding algorithm. The CIRC circuit uses two correction codes for additional correcting capability, and three interleaving stages to encode data before it is placed on a disc. CIRC also performs error correction while decoding audio data during playback.
Reed-Solomon Codes:
The Reed-Solomon code used in CIRC is an error correcting code. It is particularly suited for the CD system because its decoding requirements are relatively simple. To detect errors in the received data, for example, two syndromes, or error patterns, are calculated using decoding equations. An error results in non-zero syndromes. Further, the value of erroneous words can be determined by the difference of the weighting in decoding equations.
Cross Interleaving:
As stated in its acronym, CIRC employs cross interleaving, which is the process of rearranging data in time. Cross interleaving permits more efficient correction of errors by decoders. This is accomplished by the separation of two error correction codes by an interleaving stage. Thus, one code can check the accuracy of the other code. Another important aspect about cross interleaving is that error correction is enhanced at the expense of redundancy; that is, the amount of redundancy is not increased.
CIRC Encoding:
The objective of this encoding algorithm is the cross-interleaving of bits from the audio signal, so that two encoding stages can generate parity symbols, or data values. Error correction encoding begins with the first stage of interleaving, which is designed to assist interpolation, an error concealment technique.
First, twenty-four 8-bit symbols are applied to the CIRC encoder. A delay of two symbols is placed between even and odd samples, such that even samples are delayed by two blocks, for instance. In the case where two uncorrectable blocks occur, standard interpolation techniques can be used. Interpolation is a method used to conceal errors by using adjacent data to determine the approximate value of missing/uncorrectable data.
Next, symbols are scrambled in order to separate even and odd numbered data words. This whole process facilitates concealment, a strategy used to supply approximate data in lieu of missing or incorrect data.
The next step involves the following symbols, which represent the following: (1) P and Q are parity values, which represent ones and zeros; and (2) C1 and C2 are correction encoders capable of correcting one and two symbols, respectively.
Proceeding with the process flow, a C2 encoder accepts a 24-byte parallel word and produces 4 bytes of Q parity. Q parity is designed to correct one erroneous symbol, or up to four erasures in one word, which comprises zeros and ones. An erasure is a word that has been erased by the decoders because detection has determined its value is unreliable.
Generally, the parity symbols are placed in the center of the CIRC encoding scheme block to increase the odd/even distance. This placement occurs because it enhances interpolation in the case of burst errors, which refers to a large number of data bits lost on a medium because of excessive damage to, or obstruction on, the medium.
After the Q parity symbol is generated by the C2 encoder, cross interleaving follows. The 28 bytes, (e. g., the 24 byte parallel word plus the 4-byte Q parity word), are delayed by different periods, which are integer multiples of four blocks. As a result of this convolutional interleave, each C2 word is stored in 28 different blocks and distributed over 109 blocks.
Next, a different encoder, C1, accepts the 28-byte word from 28 different C2 words, and produces 4 additional bytes of P parity. The C1 encoder is then used to correct single symbol errors. It is also used to detect and flag double and triple errors for Q correction.
The final interleave stage introduces a fixed odd/even delay of one symbol to alternate symbols. This delay spreads the output words/values over two data blocks, in effect, preventing random errors from disrupting more than one symbol in one word. Random errors are prevented even if two adjacent symbols in one block are erroneous.
Finally, the P and Q parity symbols generated by encoders C1 and C2, respectively, are inverted to provide non-zero P and Q symbols with zero data. The inversion process assists data readout during areas with muted audio program. At the end of the CIRC encoding process, which began with a 24 eight-bit symbols, 32 eight-bit symbols leave the CIRC encoder.
CIRC Decoding:
At playback, and following de-modulation, data is sent to a CIRC decoder for de-interleaving, error detection, and correction. Essentially, the CIRC decoding process reverses many of the processing steps accomplished during encoding. The CIRC decoding process employs parity from two Reed-Solomon decoders, and de-interleaving. Upon de-interleaving, for example, errors in consecutive bits or words are distributed to a wider area to guard against consecutive errors in the storage media.
The first decoder, C1, is designed to correct random errors and to detect burst errors, (i.e. data bits lost because of a damaged/obstructed medium). It puts a flag on all burst errors to alert the second decoder, C2. Given this prior knowledge, and help from de-interleaving, C2 can adequately correct burst errors, as well as random errors that C1 was unable to correct.
During reproduction of a digital recording to a CD, the CIRC decoder accepts one frame of thirty-two 8-bit symbols. Recall that this thirty-two 8-bit symbol is comprised of 24 bytes of audio data and 8 bytes of generated parity symbols. Odd numbered symbols are delayed, and parity symbols are inverted. Each delay line has a delay equal to the duration of a single symbol. Consequently, information of even numbered symbols of a frame is de-cross-interleaved with information of the odd numbered symbols on the next frame. The de-interleaving process serves to place even and odd numbered audio symbols back into their original order by essentially re-arranging the order as read from a disc. Any sequence of errors on the disc are distributed among valid data.
In the C1 decoder, errors are detected and corrected by the 4 bytes of P parity symbols previously generated by the C1 encoder. This includes correction for short duration random errors; longer burst errors are passed along. More specifically, the C1 decoder can correct a symbol error in every word/value of 32 symbols. If there is more than one erroneous symbol, then all 28 data symbols are marked with an erasure flag and passed on. Only valid symbols, which are those adhering to C1's encoding rules, are passed along unprocessed.
Delays between decoders C1 and C2 are of unequal length, and longer than the delays at the input to the C1 decoder. This interleaving enables the C2 decoder to correct longer burst errors. Moreover, because the word arriving at the C2 decoder contains symbols from the C1 decoder that is decoded at different times, those symbols that are marked with an erasure flag get distributed among valid symbols. This situation helps the C2 decoder to correct burst errors. Symbols without an erasure flag are assumed error free and passed through unprocessed.
Contrary to the C1 decoder, where the P parity symbols are used to detect and correct errors, in the C2 decoder, errors are corrected by four Q parity symbols. If symbols are properly flagged, the C2 decoder can detect and correct single symbol errors; it can even correct up to four symbols. Burst errors arriving at C2 are also corrected, as are errors that might have occurred in the encoding process itself rather than in the medium.
Further, C2 can correct symbols that were incorrect by C1 decoding. In the event that the C2 decoder cannot accomplish correction because more than four symbols are flagged, 24 data symbols are flagged as uncorrected and passed on for interpolation, an error concealment technique. Final de-scrambling and delay is performed to assist with interpolation.
Using two correction decoders and cross interleaving helps tackle an otherwise particularly difficult error scenario. Interleaving distributes burst errors, sometimes caused by disc surface contamination, over different words for easier correction. This does not diminish the fact that correction is difficult when a burst error coincides with a random error introduced by a manufacturing defect, for example. However, since random errors are defined to be single symbol errors and any longer are burst errors, EFM (8-14 modulation) coding guarantees that a random error will never corrupt more than two symbols. The even/odd interleave guarantees that a 2-symbol random error will always appear as a single error in two different C1 words after de-interleaving. In short, this means that random errors are always correctable with the C2 decoder retaining its burst error correction capability.
CD Player Overview
The CD player contains two primary systems: an audio data processing system and a control system. Prior Art FIG. 6 depicts a block diagram of a CD player showing an audio path as well as servo and control functions. Generally, the data path, which directs modulated light from the pickup through a series of processing circuits, consists of several elements that ultimately produces a stereo analog signal. These elements of the data path include a data separator, buffer, de-interleaving RAM, error correction circuit, concealment circuit, oversampling filter, D/A converters, and output filters.
The servo and control system, in addition to a display system, directs the mechanical operation of the CD player, such as the player=s spindle drive, and the auto-tracking and auto-focusing functions. The servo, control and display system also directs the user interface to the CD player=s controls and displays.
A CD player uses a sophisticated optical read-out system to read data, control motor speed, track the pit spiral and adjust pickup positions and timings. While a spindle motor is used to rotate the disc with constant linear velocity, in another servo loop, information from the data itself determines correct rotating speed and data output rate.
User controls and their interface to the player=s circuitry is monitored by a microprocessor. A software program controls several modes of player operation. Subcode data is also used to direct the pickup to the proper disc location. For example, a time code is used to locate the start of any track.
Once data is recovered from the CD, the player must go through a series of activities to decode audio information in order to reconstruct an audio signal; namely, the EFM (eight-to-fourteen modulation) data is modulated, and errors are detected and corrected using an error correction algorithm. Additionally, using interpolation and muting, the audibility of gross errors is minimized.
Subsequent to decoding of the audio information, the digital data must be converted to a stereo analog signal. This conversion process requires one or two D/A converters and low-pass filters (in analog or digital domain).
An audio de-emphasis circuit exists in the audio output stages of every CD player. Some CDs are configured for improved signal-to-noise ratio. This configuration is accomplished by encoding the CD with an audio pre-emphasis flag in the subcode, where high frequencies on a master tape is slightly boosted (50/15 Φs characteristic). The result, on CD playback, is inverse attenuation of the disc=s high frequencies, because the player switches in the de-emphasis circuit when required, so that the signal-to-noise ratio is slightly improved.
The final output circuit is the buffer, which ensures that the CD player=s line level output is appropriate to drive necessary external amplifiers with a minimum amount of analog distortion.
Pickup Design
With respect to a player=s pickup design, a CD may contain as many as three billion pits, all orderly arranged on a spiral track. Each optical read-out system, which comprises an entire lens assembly and pickup, must focus, track and read data stored on a spiral track. The lens assembly, which is a combination of the laser beam and a reader, must be small enough to move across the underside of a disc in response to tracking information and user random-access programming. Moreover, movement of the pickup from a CD=s center to its edge must be focused despite adverse playing conditions, such as when a CD is dirty or vibrating.
Auto-Tracking
Unlike an LP, which has grooves to guide the pickup, a CD has a singular spiral pit track running from a center circle to its outer edge. The only object that touches the disc surface is an intensity-modulated laser light, which carries data and which is susceptible to obstructions, such as vibrations. Four standard methods have been designed for tracking pit spiral: (1) one-beam push-pull; (2) one-beam differential phase detection; (3) one-beam high frequency wobble; and (4) three-beam.
Auto-Focusing
The optical pickup must be precise in order to accommodate approximately 600,000 pits per second. Even the flattest disc is not perfectly flat; disc specifications acknowledge this by allowing for a vertical deflection of ∀600 Φm. In addition, a ∀2 Φm tolerance is required for the laser beam to stay focused, otherwise the phase interference between directed and reflected light is lost, along with audio data, tracking and focusing information. Therefore, the objective lens must be able to re-focus while the disc=s surface deviates vertically.
An auto-focus system, driven by a servo motor, manages this deviation, using control electronics and a servo motor to drive the objective lens. Three techniques are available for generating a focusing signal: (1) a cylindrical lens using astigmatism; (2) a knife edge using Foucault focusing; and (3) critical angle focusing.
Any pickup must perform both tracking and focusing functions simultaneously. Therefore, a completed pickup design would use a combination of the above-mentioned auto-tracking and auto-focusing techniques. Two standard pickup designs stand out from the rest when auto-tracking and auto-focusing functions are combined: (1) one-beam push-pull tracking with Foucault focusing, (hereinafter Aone-beam pickup@); and (2) three-beam tracking with astigmatic focusing, (hereinafter Athree-beam pickup@).
Both of these designs have been commercialized among manufacturers. One-beam pickups, which are usually mounted on a distal end of a pivoting arm, swings the pickup across a disc in an arc. On the other hand, three-beam pickups are mounted on a sled, which slides linearly across the disc.
The following exemplary prior art discussion will be limited to three-beam pickups only.
Three-Beam Pickup Optical Design
Prior Art FIG. 7 shows the optical path of a three-beam pickup, which uses a laser as the light source. A laser is used, rather than a bulb, for a number of reasons. First, a laser uses an optical resonator to stimulate atoms to a higher energy level that induces them to radiate in phase, a condition necessary to achieving sharper data surface focus and proper intensity modulation from the pit height.
Second, a laser light, unlike a bulb=s light, which radiates all the frequencies of a spectrum at all different phases, is composed of a single frequency and is coherent in phase. An important advantage of phase coherency is phase cancellation in the beam that is produced by disc pits, so that disc data can be read. Most CD pickups use an aluminum gallium arsenide semiconductor laser with a 0.5 milliwatt optical output that radiates a coherent-phase laser beam with a 780 nanometer wavelength, because the beam is comprised of near-infrared light.
Referring to Prior Art FIG. 7, a laser diode is positioned adjacent the focal point of a collimator lens with a long focal distance, for the purpose of making the divergent light rays parallel. A monitor diode (not shown) is also placed adjacent the laser diode in order to control power to the laser. The monitor diode stabilizes the laser=s output in two important ways; first, by compensating for temperature changes so as to prevent thermal runaway; and second, by conducting current in proportion to the light output of the laser.
The three-beam pickup is so termed because it uses three beams for tracking and reading a CD. To generate these beams, a laser light first passes through a diffraction grating, which resembles a screen with evenly-spaced slits of a few laser wavelengths apart. As the beam passes through the grating, the light diffracts into fringes of parallel light beams. When the collection of these beams is re-focused, the collection appears as a single, bright centered beam with a series of successively less intense beams on either side of the center beam.
It is this diffraction pattern that actually strikes the CD, where the center beam is used for both reading data and focusing. In a three-beam pickup, two of the series of successively less intense beams, or two secondary beams, are used for tracking only. In a one-beam pickup, data reading, focusing and tracking is accomplished with just one beam.
Another element in the three-beam optical design is the polarization beam splitter, or PBS, which consists of two prisms having a common 45 degree facing that acts as a polarizing prism. The purpose of the PBS is to direct the laser light to the disc, and to angle the reflected light (from the disc) to the photosensor. In some designs, a half-silvered mirror is used.
In Prior Art FIG. 7, the collimator lens is shown as following the PBS, even though it can precede the PBS in other designs. Once the light exits the collimator lens, it then passes through a quarter-wave plate (QWP). The QWP is an anisotropic material that exhibits properties with different values when measured in different directions, so that when light passes through the QWP, it rotates the plane of polarization of each passing light beam. This rotation is required to make the PBS work.
The anisotropic quality of the quarter-wave plate is equally important to the process occurring on the right-hand side of the plate. Light passing through the QWP to the CD, will be reflected from the CD back again through the QWP and become polarized. More importantly, the light is polarized in a plane at right angles to that of the incident light.
In other words, the reflected polarized light re-entering the quarter-wave plate (from right to left) will pass through the collimator and strike the polarization beam splitter. Because the polarization beam splitter passes light in one plane only (e.g., horizontally) but reflects light in the other plane (e.g., vertically), the PBS will properly deflect the reflected beam toward the photodiode sensor to read the digital data.
The final optics element in the path to the CD is the objective lens. The objective lens is used to focus laser beams into a convergent cone of light onto the CD=s data surface, taking into account the refractive index of the polycarbonate substrate of the disc. Convergence is a function of the numerical aperture (NA) of the lens, with most pickups using an objective lens having an NA of about 0.5.
As mentioned earlier, the laser beam=s size on the outer surface of the CD=s transparent polycarbonate substrate is approximately 800 micrometers in diameter. Since the refractive index of the substrate is 1.55 and its thickness is 1.2 millimeters, the laser beam=s size is narrowed to 1.7 micrometers at the reflective surface, a size slightly wider than the pit width of 0.5 micrometer and comparable in width to the light=s wavelength.
When the laser beam strikes a land, (the smooth surface between two pits), light is almost totally reflected. When the light strikes a pit (viewed as a bump by the laser), diffraction and destructive interference cause less light to be reflected.
In short, all three intensity-modulated light beams pass through the objective lens, the QWP, collimator lens, and the PBS. Before hitting the photodiode, they pass through a singlet lens and a cylindrical lens.
In any optical pickup system, automatic focusing is an absolute prerequisite. Disc warpage and other irregularities causes vertical deflections in the CD=s data surface. Such movement would place the data out of the pickup=s depth of focus, essentially making it impossible for the pickup to distinguish between pit height and land phase differences.
The unique properties of astigmatism are used to achieve auto-focusing in a three-beam CD player. This is illustrated in Prior Art FIG. 8.
The cylindrical lens, (see Prior Art FIG. 7), which prefaces the photodiode array, detects an out-of-focus condition. The condition is directly related to the distance between the objective lens and the CD=s reflective surface. As this distance varies, the focal point changes, and the image projected by the cylindrical lens changes its shape. The inter-relationship of the above elements is illustrated in Prior Art FIG. 8.
Changes in an image on the photodiode generates a focus correction signal. For example, when the distance between the objective lens and the CD decreases, the image projected by the lens moves further from the cylindrical lens, and the pattern becomes elliptical. Conversely, when the distance between the objective lens and the CD increases, the image projected by all lenses (e.g., the objective lens, an intermediate convex lens and the cylindrical lens) moves closer to the lens. However, the elliptical pattern that is formed is now rotated 90 degrees from the first elliptical pattern.
In the third and final scenario, which is when the disc surface lies exactly at the focal point of the objective lens, the image reflected through the intermediate convex lens and cylindrical lens is unchanged, and a circular spot strikes the center of the photodiode.
An important aspect of the three-beam auto-focus system is correction voltages. A photodiode uses a laser beam=s intensity level to generate a focus correction voltage, which in turn generates a control signal. These electrical signals control the mechanical motion of a servo motor, which is responsible for moving the objective lens along an optical axis in response to any vertical disc motion. Servo-controlled movement of the objective lens during disc motion results in automatic focusing.
Prior Art FIG. 9 illustrates a typical servo motor used to move the objective lens in the optical path. The servo motor consists of a coil and magnet structure generally used in loudspeakers.
Operation of a CD player begins when a CD is first loaded into the player. Technically, an electrical control signal is sent into the optical pickup system, which causes the laser to turn on, and the objective lens to move vertically until a focus condition is reached.
Then, the auto-focusing system takes over, except if two negative situations occur. If no CD is detected, the automatic focusing system tries again, and cuts off if it fails to detect a CD again. If the auto-focus is inoperative, such as when the CD tray is open, the system pulls back the objective lens to prevent damage to the lens or CD. Otherwise, the automatic focusing system performs its operation smoothly by keeping the pickup properly positioned beneath the spinning disc, in effect maintaining focus to within a tolerance of approximately ∀0.5 micrometers.
Content Scrambling System
Currently, encryption for data media, such as DVDs, involves one key. It is a fairly simple 40-bit scheme. There is good authentication of the platform, which is performed by various key exchanges within the mechanisms between the source drive and the actual platform decrypting the data.
A content scrambling system (CSS) is included in every DVD player. CSS is a method of encrypting a disc that the information technology (IT) and motion picture industries agreed upon. In order to be licensed to manufacture DVD players, a company is required to obey certain rules pertaining to the uses (and non-uses) that a platform can perform, as part of a license agreement.
While the present invention is not required to incorporate the CSS encryption system, it could be one level of encryption, if a multi-level encryption is employed. Audio information is generally encrypted prior to being burned into a disc, such as a CD. Hence, there is no plain text; encrypted information only is contained on a CD. So, if a user seeks to access information contained on the CD, whether for listening or copying purposes, the user would have to decrypt the data in order to hear sensible audio data.
In general, existing ideas in the field appear to bury authentication keys within encrypted information that is burned into the disc. Authentication keys are buried using various authentication processes, which verify that the platform device—whether a computer, CD player, DVD player, or the like—is a licensed device and, consequently, obeys certain copyright rules. Eventually, the licensed device uncovers the buried authentication key(s) and decrypts the data contained on the disc. So, the system needs to find the key before being eligible for decrypting the audio data.
The following prior patents represent the state of the art of preventing unauthorized copying of data, and are all hereby incorporated by reference:
U.S. Pat. No. 4,811,325 to Sharples, Jr. et al. discloses a high speed copying of audio programs on optical CDs. The master CD is encoded using Adaptive Delta Modulation (ADM).
U.S. Pat. No. 4,879,704 to Takagi et al. prevents copying of an optical disc. Data is stored in a record protected area and in a record unprotected area, where each such sector has a representative address that helps to determine whether the data is in the record protected area or in the record unprotected area. Only data from the record unprotected area with an appropriate address can be copied.
U.S. Pat. No. 4,937,679 to Ryan discloses a video recording and copy prevention system. The video signal includes a copy-protect signal. Designated detectors detect the presence of copy-protected signal(s) and inhibit copying of such signals. A video correlate enables one to playback a copy-protected program for viewing only and generates an inhibit signal to prevent copying of a copy-protected signal.
In U.S. Pat. No. 4,975,898 to Yoshida, an erasing program erases the non-rewritable portion so that it cannot be copied on a copy disc during unauthorized copying of an optical disc.
U.S. Pat. No. 5,319,735 to Preuss et al. uses a digital code signal embedded with the original audio signal. The digital code gets transferred to the copy disc.
In U.S. Pat. No. 5,412,718 to Narasimhalu et al., non-uniformities and their attributes in the storage medium is used as a unique signature. This signature is used to derive a key for encrypting the information on the storage medium. During copying, the signature gets mutated and the information cannot be decrypted. During authorized copying, the information is decrypted by generating a key from the signature of the distribution medium.
In U.S. Pat. No. 5,418,852 to Itami et al., data is stored in a user accessible area and in a user inaccessible area, which are both compared to determine the authenticity of the recording medium.
In U.S. Pat. No. 5,513,260 to Ryan, copy-protected CDs have authenticating signature recorded on them. An authentication signature is obtained by a deliberately induced radial position modulation giving an error voltage corresponding to the elliptical errors. When playing the CD, the signature causes the player to correctly decrypt the program whereas, when playing an unauthorized copy of the CD, the absence of the signature is detected and false data is generated and the player does not play.
U.S. Pat. No. 5,538,773 to Kondo discloses the recording of data together with a cipher key information for copy protection.
U.S. Pat. No. 5,570,339 to Nagano discloses a system that converts data to digital data, which is then FM modulated with key information to vary the widths of the pits at the time of recording. During reproduction, the data is read out and if the key information is determined to be missing, copying is prevented.
U.S. Pat. No. 5,608,717 to Ito et al. discloses a CD-ROM that has a character/graphic pattern for copy protection. Password and information on the position of the character/graphic pattern bearing area of the CD-ROM are stored beforehand in a memory included in the CD-ROM=s controller of the playback system. The CD-ROM controller, therefore, will have the means for deciphering the enciphered password. Data are modulated by the EFM modulation method into bits of predetermined width and height having values corresponding to the EFM.
U.S. Pat. No. 5,608,718 to Schiewe discloses an optical disc having shallow pits bearing an identification/logo/watermark. The lands and pits are of different lengths for identification/authorization purposes when copying a CD.
U.S. Pat. No. 5,636,276 to Brugger discloses the distribution of digital music with copyright protection. An encryption table is embedded in the music CD player and includes a decryption module that uses the encryption table for authorized playing of music/information.
U.S. Pat. No. 5,636,281 to Antonini discloses an authorized access that uses mingling of data elements of the program memory to be protected according to a secret order. To use this memory, a transconding device is used. The transconding device is in the form of a memory containing several tables, only one of which gives the right transconding data elements.
The problem with one or more of the above-mentioned conventional encryption/decryption system is that a pirate or hacker seeking to hack into the encryption process on a disc could do so by playing the encrypted music, finding the decryption key, which is buried, mixed and interleaved with the audio data or the encrypted audio data, and using that key to decrypt the audio on the disc.
In other words, accompaniment of the decryption key within the audio data lends itself to discovery, even if the audio data is played in an encrypted form. A hacker could obtain decryption key(s) even if the encrypted audio data was placed onto an unlicensed computer platform having a DVD ROM drive that did not obey copyright protection rules, because if the audio is later played back, the key would be output along with the encrypted audio data.
An additional problem in one or more of the prior art references is that keys specific to, or derived from, the physical construction of the CD are not constructed or determined in a manner that is difficult to detect by a hacker. A further problem in the prior art is that the physical characteristics of the CD which are used to derive a key for authorized copying, are transferred in the audio and may be accessible to the hacker.
Yet another problem in one or more of the prior art references is that the solutions proposed therein require significant additional hardware and/or software to be implemented. That is, these prior art techniques do not take advantage of existing hardware/software within the CD or DVD player that can be used effectively to prevent unauthorized copying.
Yet another problem in one or more of the prior art references is that the solutions proposed therein are expensive, and incompatible with existing CD or DVD players. Hence, current solutions to unauthorized copying are difficult and impractical in their implementation.
Yet another problem in one or more of the prior art references is that the solutions proposed therein are limited to CD and/or DVD players, and does not consider or structure such techniques when data is transmitted from, to, and/or, or over local and/ore global networks, such as the Internet.